Electrospinning of polymer solutions under a high humidity environment is studied. It is shown that the porous morphology formed after phase separation can be preserved or collapsed depending on three factors: phase demixing time, physical gelation, and viscoelastic properties of the polymer-rich phase. Fibers with rough surfaces and nonporous cross sections are produced when poly(ether imide)/dimethylformamide is electrospun. This is due to accelerated vitrification-related gelation of the water/dimethylformamide/poly(ether imide) system on the fiber surface. Similar phase behavior can be expected for a ternary system based on polystyrene. However, the new system results in fibers with smooth surfaces and porous cross sections. This discrepancy can be resolved by considering delayed gelation as well as lower elastic and loss moduli of the polymer-rich phase in the latter system. Further evidence is also provided by poly(ether sulfone) and polysulfone. Crystallization-induced gelation observed for poly(vinylidene fluoride) fibers can well account for the obtained morphology. However, crystallization-induced gelation cannot lock in the fiber morphology similar to the vitrification-related gelation.